The primary secretion is produced by acinar cells in the secretory end pieces (or acini) and is modified by duct cells as saliva passes through the ducts. The primary secretion is isotonic, and the concentration of the major ions is similar to that in plasma. Secretion is driven predominantly by Ca++-dependent signaling, which opens apical Cl− channels in the acinar cells. Cl− therefore flows out into the duct lumen and establishes an osmotic and electrical gradient. Because the epithelium of the acinus is relatively leaky, Na+ and water then follow across the epithelium via the tight junctions (i.e., via paracellular transport). Transcellular water movement may also occur, mediated by aquaporin 5 water channels. The amylase content and rate of fluid secretion vary with the type and level of stimulus. As the fluid passes along the ducts, the excretory and striated duct cells modify the ionic composition of the primary secretion to produce the secondary secretion. The duct cells reabsorb Na+ and Cl− , and secrete K+ and HCO3 − into the lumen. Na+ is exchanged for protons, but some of the secreted protons are then reabsorbed in exchange for K+ . HCO3 − on the other hand is secreted only in exchange for Cl−, thereby alkalinizing salivary secretion.
At rest, final salivary secretion is hypotonic and slightly alkaline. The alkalinity of saliva is important in restricting microbial growth in the mouth, as well as in neutralizing refluxed gastric acid once the saliva is swallowed. When salivary secretion is stimulated, there is a small decrease in the K+ concentration (but it always remains above plasma concentrations), the Na+ concentration increases toward plasma levels, and Cl− and HCO3 − concentrations increase, thus the secreted fluid becomes even more alkaline (
Fig. 28.2 ). Note that HCO3 − secretion can be directly stimulated by the action of secretagogues on duct cells. The duct epithelium is relatively tight and lacks expression of aquaporin, and therefore water cannot follow the ions rapidly enough to maintain isotonicity at moderate or high flow rates during stimulated salivary secretion. Thus with
CHAPTER 28 The Cephalic, Oral, and Esophageal Phases of the Integrated Response to a Meal 1.0 2.0 3.0 4.0 Flow of saliva (mL/min) Na+ Saliva Plasma Cl– K+ Cl– Na+ K+ HCO– 3 HCO– 3
Amylase-containing PRIMARY SECRETION (nearly isotonic; levels of Na+, K+, Cl–, and probably HCO3 similar to plasma)
Modification of ionic content • Fig. 28.2 A, The composition of salivary secretion as a function of the salivary flow rate compared with the concentration of ions in plasma. Saliva is hypotonic to plasma at all flow rates. [HCO3 −] in saliva exceeds that in plasma except at very low flow rates. B, Schematic representation of the two-stage model of salivary secretion. The primary secretion containing amylase and electrolytes is produced in the acinar cell. The concentration of electrolytes in plasma is similar to that in the primary secretion, but it is modified as it passes through ducts that absorb Na+ and Cl− and secrete K+ and HCO3 − .
an increase in secretion rate, there is less time for ionic modification by the duct cells, and the resulting saliva more closely resembles the primary secretion and therefore plasma. However, [HCO3 −] remains high because secretion from duct cells and possibly acinar cells is stimulated (see
Fig. 28.2).